In the art of growing thin films, it is known to expose a substrate to an oblique incident vapor flux in conditions of limited adatom diffusion and thus grow a columnar microstructure on the substrate.
The optical properties of the resulting microstructure are dependent in part on the material used, the porosity of the microstructure and the orientation of the columns of the thin film.
Hamaguchi et al, U.S. Pat. No. 4,874,664, describe lateral shifting or rotation of the position of the substrate in relation to the vapor flux to create uniform film growth and film layers that have columns with different orientations in the different layers. In Hamaguchi et al, the entire substrate is rotated in between periods of exposure of the substrate to vapor flux, or the substrate is laterally shifted during exposure to vapor flux.
A paper of Azzam, "Chiral thin solid films", Appl. Phys. Lett. 61 (26) Dec. 28, 1992, has proposed rotation of the substrate while it is exposed to the oblique incident vapor flux to generate a helical microstructure having helicoidal bianisotropic properties. The proposed rotation of the substrate is about an axis perpendicular to the surface of the substrate, which is referred to in this patent document as rotation about the azimuth, or variation of the azimuthal angle.
The inventors have proposed improvements to the methods of Azzam and Hamaguchi, as described in "First thin film realization of a helicoidal bianisotropic medium", Kevin Robbie and Michael J. Brett, Akhlesh Lakhtakia, J. Vac. Sci. Technol. A 18(6), Nov/Dec 1995, p. 2991-2993.
In these references, the relationship of porosity to column angle is fixed, and there has been no way to vary porosity while maintaining the column angle fixed. The present invention seeks to overcome this limitation in the earlier references.